Giant Killer and War Caster

[RingoBongo]

Hunter ranger feature Giant Killer:

"When a Large or larger creature within 5 feet of you hits or misses you with an attack, you can use your reaction to attack that creature immediately after its attack, provided that you can see the creature."

War Caster feat, last bullet point:

"When a hostile creature’s movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature."

So, instead of just hitting the bad guy back with a regular attack, can I use booming blade to hit back?

I can't find any official ruling on this subject... Please help!

[Hytheter]

The answers are all in your post. War Caster specifies "opportunity attack" which Giant Killer isn't, and also specifies "creature's movement" which isn't what triggers Giant Killer.

[RingoBongo]

Yeah I guess I was just dissatisfied by being unable to find any exact reference to this question on the internet.

Semantics matter I suppose... Because an enemy doesn't "move" when it attacks and a "using ones reaction to attack" is not the same as an AoO (even though that's basically it's definition).

[Lunali]

There are a few places where a distinction is made between "attacks of opportunity" and other reactions that allow you to make an attack against an enemy. The most obvious example of this is the sentinel feat. It both gives you the ability to attack enemies as a reaction, and applies extra effects to "attacks of opportunity" but since the reaction attack isn't described as an "attack of opportunity," using it doesn't apply the extra effect.